1. Field of the Invention
The present invention relates to a photopolymerizable composition. More particularly, the present invention relates to a photopolymerizable composition usable especially as a solder resist for coating a printed circuit board which can produce a coated film possessing an excellent rust prevention for a copper-foiled pattern in addition to the superior characteristics such as thermal resistance and mechanical and electrical characteristics.
2. Description of the Prior Art
Of late, a solder resist has been employed in soldering a printed circuit board. This solder resist is such as to be coated on a printed circuit board for the purposes of reducing the amount of solder to be used; preventing the occurrence of solder's bridging; protecting the board from oxidation or from gases such as sulfur dioxide, hydrogen sulfide and the like; improving the insulating characteristic of the board; preventing the copper-foiled pattern from rusting; and further preventing the board from warping. In view of the facts that the solder resist, as mentioned above, acts as a soldering mask for the printed circuit board in soldering and further acts as a coating film for protecting the board permanently, the solder resist must satisfy various requirements. That is, the solder resist must have the following characteristics: (1) thermal resistance and anti-solubility during hot process in soldering, (2) stiff close adherence and adhesive property to the printed circuit board, (3) excellent electric characteristics, (4) a small degree of deterioration even under the high temperature and humidity conditions and the like. In general, since the solder resist is designed to be coated after a desired pattern on the print circuit board by the screen printing method, it also becomes necessary for the solder resist to possess the characteristics required for a printing ink such, for instance, as the print-definition and fluidity for permitting the accurate regeneration of the image of the screen.
As the solder resists there have usually been used the ink compositions of thermo-setting type which are consisted essentially of thermo-setting resins such as alkyd-melamine resin, epoxy resin and the like and further contain organic solvents. Of late, however, there have been used, as the solder resists, photopolymerizable ink compositions with the intention of shortening the setting time, improving the working properties and improving the environmental problems such as air pollution caused by the use of organic solvents or the like. This photopolymerizable solder resist is consisted essentially of the acrylate oligomer represented by epoxy acrylate and urethane acrylate and of the vinyl monomer such as acrylate, and can be cured readily by exposing it to activated radiation of ultraviolet ray, electronbeam or the like. U.S. Pat. No. 3,772,062, U.S. Pat. No. 4,014,771, U.S. Pat. No. 4,271,258 and the like is reporting that epoxy acrylate is superior in the properties of light sensitivity, hardness, solvent resistance, thermal resistance and the like. Referring to urethane acrylate, whilst, U.S. Pat. No. 4,072,770 and the like report that urethane acrylate is superior in flexibility, adhesive property and the like.
The above photopolymerizable composition is obtained in the manner that the oligomer containing an acryloyl group or a methacryloyl group and the monomer containing an acryloyl group or a methacryloyl group are cross linked by the radical reaction of vinyl groups and cured. As is generally known, however, this curing reaction is followed by a very hard curing contraction, thereby deteriorating the close adherence and adhesive property between the resulting coated film and printed circuit board. Further, there remains such a defect that the unreacted monomer present in the cured coated film corrodes the copper-foiled pattern of the printed circuit board, which is remarkable especially when left standing at high temperature and humidity for a long period of time.
U.S. Pat. No. 4,320,189 proposes that a halogen atom (such as melamine, guanamine, acetoguanamine, benzoguanamine, ethyldiamino-s-triazine, 2,4-diamino-s-triazine, 2,4-diamino-6-tolyl-s-triazine or the like)-, an alkyl group-, amino group-, alkylamino group- or allyl group- substituted s-triazine compound be added to the photoresist composition in order to improve the adhesive property of the coated film to the board. Even if this composition be utilized as the solder resist in this instance, the adhesive property of the resulting coated film to the copper surface may be improved somewhat, but the rust prevention of copper foil is insufficient. As melamine is not well dispersive, moreover, there can not be obtained a smooth coated film.
In order to improve the rust prevention of copper foil, still more; it has usually been tried to add imidazoles such as benzimidazole, imidazolecarboxylic acid, dimethylimidazole, isopropylimidazole, undecylimidazole, 2-methyl benzthiazole, 2-mercaptobenzimidazole, benztriazole, 1-chlorobenzotriazole and the like; and their derivatives to the ink composition for solder resist, but no satisfactory effect has been achieved yet.